1. Field of the Invention
Embodiments of the invention described herein pertain to the field of marine transportation of liquefied cryogenic hydrocarbons. More particularly, but not by way of limitation, one or more embodiments of the invention enable an apparatus, system and method for the capture, utilization and sendout of latent heat in boil off gas onboard a cryogenic storage vessel.
2. Description of the Related Art
It is often advantageous to transport hydrocarbons that are normally in gaseous form at ambient conditions as sub-cooled liquids. For example, although many hydrocarbons are typically transported from the location produced to the location consumed as a gas by pipeline, there are circumstances where doing so may not be feasible. For example, the location of production and the location of demand may be separated in such a manner that ocean transport aboard a vessel over much of the distance to market is more economically feasible than transport via pipeline. Without an effective way to transport the gas to a location where there is a commercial demand, opportunities to monetize the gas may be lost. This may be true for many types of hydrocarbons, such as natural gas, ethane, pentane or ethylene, which are often transported across the ocean in a liquefied state by vessel.
Liquefaction of gaseous hydrocarbons facilitates storage and transportation of the hydrocarbon. For example, liquefied natural gas (“LNG”), largely consisting of methane mixed with other hydrocarbon components, takes up only about 1/600 of the volume that the same amount of natural gas does in its gaseous state. A liquefied hydrocarbon is produced by cooling the hydrocarbon below its boiling point (for natural gas, about −160° C. at atmospheric pressure, depending on cargo grade; for ethylene about −104° C., for ethane about −89° C.). The liquefied hydrocarbon may be transported and stored in cryogenic containers slightly above atmospheric pressure. Upon reaching the location of intended use, the liquefied hydrocarbon may be converted back to its gaseous form by adding heat and thereby raising the temperature above its boiling point.
Liquefied hydrocarbons are typically stored onboard cryogenic storage vessels, such as liquefied petroleum gas (“LPG”) carriers, which are designed to carry mainly butane, propane, butadiene, propylene vinyl chloride monomer or anhydrous ammonia; LNG carriers, which are designed to carry LNG (mostly methane); ethylene carriers, which may also carry LPG; and ethane carriers, which may also carry LNG. In each case, the liquefied gas is stored onboard a vessel in insulated storage tanks to minimize ambient heat ingress and the accompanying rise in temperature and pressure it causes. Nonetheless, heat enters the cargo tanks due to the large temperature differential and is absorbed by the cargo, raising the cargo's temperature and pressure.
In order to control tank pressures, heat is removed from the cargo by allowing a portion of the cargo to boil off, which releases the latent heat of vaporization contained in the vapor generated from the liquid remaining. The resultant vapor, commonly referred to as “boil off” or “BOG” (Boil-Off Gas) is continually removed from the tanks in order to maintain a safe operating pressure and cargo temperature within the tanks.
BOG is commonly used as a fuel in a vessel's power plant to provide for the vessel's energy needs. For example, many vessels make use of a steam turbine as the main propulsion engine. In such instances, BOG may be burned in steam boilers to produce steam for the steam turbines. In other power plants, the BOG is consumed in dual fuel diesel engines driving electrical generators.
However, in many cases, the energy available in the BOG exceeds the vessel's requirements. In such instances, the excess BOG must be disposed of, and is wasted. Typically, excess steam generated by the ships boilers is dumped to the main condenser where it is condensed and the heat removed is transported overboard with the circulating water. Alternatively, ships may be outfitted with gas combustion units (“GCU”) which burn the excess BOG.
To avoid wasting BOG, new-build vessels have been fitted with nitrogen expansion reliquefaction plants or suction drum-type recondensers, the latter of which incorporate a recondenser in the upper portion of a suction drum. However, reliquefaction plants and suction drum recondensers are capital intensive and utilize separate refrigeration systems for reliquefaction. Reliquefaction plants, in addition, require large amounts of electrical power, and current recondensers may require a minimum of 80 to 85 mmscf/d sendout to operate in order to recondense all BOG fed to them during operation. As a result, reliquefaction plants and recondensers may not be feasible or appropriate as retrofit solutions for existing vessels because they require significant additional equipment and downtime for installation.
It has also been proposed that BOG, which has been condensed, be returned to the liquefied gas cargo tank. However, with this approach, the condensed BOG is at a higher pressure and temperature than the ambient pressure and temperature in the cargo tank. When the BOG is reintroduced into the tank, the BOG goes from a saturated condition to a lower pressure condition outside the saturation range for the temperature of the cargo tank. As a result, the condensed BOG flashes off at let-down, effectively returning much of the condensed BOG back to vapor in the cargo and replacing BOG removed from the tank.
In some instances, cryogenic storage vessels are equipped with regasification facilities and special arrangements, including gas arms, which provide for the regasification of a liquefied hydrocarbon aboard the vessel and the discharge of the gas to a pipeline. This has certain advantages, in that the regasification facility travels with the vessel. This can make it easier to accommodate gas demand that is seasonal or otherwise varies from location to location. Because the regasification facility travels with the vessel, it is not necessary to provide a separate storage and regasification facility, either onshore or offshore, at each location at which liquefied gas may be delivered.
In regard to regasification vessels and onshore regasification facilities that receive liquefied gas from seagoing vessels, wasted BOG directly affects the overall efficiency of the regasification facility. The efficiency is typically measured as the amount of liquefied gas actually delivered as compared to the amount loaded onboard the vessel and is expressed as a percentage.
Conventional techniques for conserving BOG are not well suited as retrofit solutions for existing steam powered cryogenic storage vessels to improve their efficiency. In regard to newbuild vessels reliquefaction plants may not be cost effective. Therefore, there is a need for an apparatus, system and method for capturing, utilizing and sending out the latent heat contained in the BOG onboard a cryogenic storage vessel.